1. Field of the Invention
The present invention relates to a display device, and more particularly, to a backlight unit and a liquid crystal display device using a backlight unit.
2. Discussion of the Related Art
As the information age progresses, flat panel display (FPD) devices having light weight, thin profile, and low power consumption characteristics are being developed and commonly used as a substitute for cathode ray tube (CRT) devices. Generally, display devices may be classified according to their ability for self-emission, and may include emissive display devices and non-emissive display devices. The emissive display devices display images by taking advantage of their ability to self-emit light, and the non-emissive display devices require a light source since they do not emit light by themselves. For example, plasma display panel (PDP) devices, field emission display (FED) devices, and electroluminescent display (ELD) devices are commonly used as emissive display devices. Liquid crystal display (LCD) devices, which may be categorized as non-emissive display devices, are commonly used in notebook and desktop computers because of their high resolution, capability of displaying color images, and high quality image display.
A LCD module of the LCD device includes an LCD panel for displaying images to an exterior and a backlight unit for supplying light to the LCD panel. The LCD panel includes two substrates facing and spaced apart from each other, and a liquid crystal material interposed therebetween. Liquid crystal molecules of the liquid crystal material have a dielectric constant and refractive index anisotropic characteristics due to their long, thin shape. In addition, two electric field generating electrodes are formed on the two substrates, respectively. Accordingly, an orientation alignment of the liquid crystal molecules can be controlled by supplying a voltage to the two electrodes, wherein transmittance of the LCD panel is changed based on polarization properties of the liquid crystal material. However, since the LCD panel is a non-emissive-type display device, an additional light source is required. Thus, a backlight unit is disposed under the LCD panel, wherein the LCD device displays images using light produced by the backlight unit. In general, backlight units may be classified into two types according to a disposition of the light source, such as side-type and direct-type. As display areas of the LCD devices become increasingly larger, direct-type backlight units including a plurality of light sources have become commonly used to provide high brightness.
FIG. 1 is a schematic cross sectional view illustrating a backlight unit of an LCD module according to the related art.
In FIG. 1, an LCD module 1 includes a main support 14 supporting a LCD panel 2 of the LCD module 1, a top case 10 covering the main support 14 and an edge of the LCD panel 2, and a backlight unit 30 under the LCD panel 2. The backlight unit 30 includes an optical element 31 and a light guide plate 24.
Upper and lower polarizers 42 and 40 are disposed on upper and lower surfaces of the LCD panel 2, respectively. Although not shown, the LCD panel 2 includes an upper substrate, a lower substrate facing the upper substrate, and a liquid crystal layer interposed between the upper and lower substrates, wherein the lower substrate includes a plurality of gate lines and a plurality of data lines having a matrix type, and a plurality of thin film transistors connected to each of the plurality of gate lines and plurality of data lines.
More specifically, the top case 10 is bent to cover sides of the main support 14 and the edge of the LCD panel 2, and the main support 14 and the top case 10 are combined by a screw (not shown). Although the main support 14 generally is formed as a mold type, it may further include a metallic material having a characteristic of protection against heat, such as aluminum (Al), with respect to a high brightness television or monitor. A backlight unit 30 is disposed under the main support 14 and includes a backlight lamp 20 as a light source, a lamp housing 16 covering the backlight lamp 20, a light guide plate 24 converting incident light from the lamp 20 into plane light, a reflector 26 on an backside of the light guide plate 24, and the optical element 31 over the light guide plate 24. The optical element 31 has a diffusion sheet 32, an upper prism sheet 36 and a lower prism sheet 34 sequentially laminated over the light guide plate 24. The LCD panel 2 is disposed over the optical element 31, wherein the LCD panel 2 is substantially disposed over the optical element 31 through a protrusion 14a of the support main 14.
The backlight lamp 20 includes, for example, a cold cathode fluorescent lamp (CCFL) and light emitting from the lamp 20 enters the light guide plate 24 through an incident surface defined as a side portion of the light guide plate 24.
The lamp housing 16 reflects light emitting from the backlight lamp 20 toward the incident surface of the light guide plate 24 because the lamp housing 16 includes a reflecting portion on an inner surface thereof. In addition, the light guide plate 24 is manufactured to have a sloped backside and a horizontal front side. The reflector 26 has a role of reducing light loss by reflecting light toward the light guide plate 24 such that incident light reflects through the backside of the light guide plate 24. As a result, when the light emitting from the backlight lamp 20 enters the light guide plate 24, the light is reflected with a predetermined angle on the sloped backside, thereby uniformly progressing with respect to an entire surface of the light guide plate 24. At this time, the light emitted to the lower and side surfaces of the light guide plate 24 enters toward the entire surface by reflecting to the reflector 26. In other words, the light emitting from the light guide plate 24 is diffused on an entire area by the diffusion sheet 32.
In addition, when the incident light is perpendicularly disposed with the LCD panel 2, light efficiency increases. Therefore, the upper and lower prism sheets 36 and 34, may be laminated as two pieces to be perpendicularly disposed with the angle of the light emitting from the light guide plate 24 for the LCD panel 2 in order to increase light efficiency. The light through the diffusion sheet 32 may enter the LCD panel 2 through the upper and lower prism sheets 36 and 34.
However, there is a gap A in an assembly structure of the mentioned backlight unit 30, an interval between the optical element 31 and the LCD panel 2 forms the gap A.
As a result, the gap A leads to a bright line in a portion of the light guide plate 24 where light enters due to the sloped side of the optical element 31. That is, the bright line is shown in the LCD panel 2 adjacent to the lamp 20, wherein the bright line leads to problems such that brightness uniformity of the whole LCD panel 2 is reduced and the whole brightness is decreased. More particularly, in the case that the light guide plate 24 includes a prism pattern, the bright line phenomenon is exacerbated.